Concerns about the impact of the disposal of batteries on the environment have led to the development and improvement of rechargeable cells, also referred to as secondary cells by those skilled in the art.
Non-aqueous alkali metal secondary cells typically include an anode of an alkali metal, such as lithium, potassium or sodium, an electrolyte prepared from an alkali metal salt dissolved in an organic solvent, and a cathode of an electrochemically active material, typically a chalcogenide of a transition metal. During discharge, alkali metal ions from the anode pass through the electrolyte to the cathode where the ions are taken up with a simultaneous release of electrical energy. During charging, the flow of ions is reversed so that alkali metal ions pass from the cathode through the electrolyte and are plated on the anode.
During each discharge/charge cycle, small amounts of the alkali metal and electrolyte are consumed by chemical reactions at newly created surfaces on the alkali metal of the anode. This reaction condition is further aggravated by the tendency of the alkali metal to form dendrites as it is plated back onto the anode. The dendrites continue to grow until they eventually contact the cathode, thereby causing the cell to fail. Furthermore, the alkali metal may not cohesively plate onto the anode during the charge cycle, resulting in the formation of spongy deposits near the surface of the anode. The spongy deposits are not in electrically conductive contact with the anode and eventually may adversely affect the capacity of the cell.
This consumption of the alkali metal may be minimized by providing a sheet-like microporous separator on the surface of the alkali metal and by applying substantial pressure on the separator and the anode, so that the alkali metal is deposited on the anode in the form of a layer, thereby preventing the growth of dendrites and spongy deposits. Typically, pressure is applied as an interelectrode pressure, also referred to by those skilled in the art as "stack pressure". However, only cells with cylindrical symmetry are capable of withstanding the stack pressure with a thin metal casing. Rectangular and coin-shaped cells would require very thick metal casings in order to withstand the stack pressure without excessive flexing. However, the cell would then be significantly larger and more expensive to produce.
Furthermore, microporous separators which are capable of preventing dendritic penetration and withstanding the applied stack pressure are typically very expensive. However, there is still a risk that the microporous separator will be punctured by dendritic growth. As a result, long recharge times are required to reduce the risk of puncture. Unfortunately, the risk of puncture increases with repeated chargings even at low rates, thereby limiting the number of discharge/charge cycles which may be obtained during the life of the cell.
Even when a microporous separator and the appropriate stack pressure are used, a small percentage of the alkali metal is still consumed during each discharge/charge cycle. Thus, in order to attain a practical cell life, a substantial excess of the alkali metal is required in the cell, thereby significantly increasing the cost and size of the cell.
Moreover, alkali metals are extremely reactive and have low melting points. Accordingly, excess heat generated during extended operation, especially in relatively large cells, may lead to melting of the anode. Such melting may not only render the cell inoperative, but could also lead to an undesirable reaction between the alkali metal and electrolyte and to direct contact between the molten alkali metal and the electrochemically active material of the cathode, resulting in a vigorous reaction that could rupture the cell casing.
Thus, there is a need for a secondary cell which will provide the advantages provided by an alkali metal anode, but which will not have the drawbacks associated with this type of cell. One approach has been to replace the alkali metal anode with a carbonaceous anode formed by a carbonaceous material intercalated with alkali metal ions to form compounds of the formula M.sub.x C, wherein M represents an alkali metal. In operation of the cell, alkali metal ions pass from the intercalated carbonaceous material through the electrolyte to the cathode. When the cell is recharged, the alkali metal ions are transferred back to the anode for re-intercalation with the carbonaceous material, thereby preventing the formation of dendrites or spongy deposits. Furthermore, melting of the anode cannot occur, even under extended periods of operation, because the alkali metal of the anode is not in a metallic form.
Suitable carbonaceous materials include graphite, coke, carbon fibre, pyrolytic carbon, non-graphitizable carbon and chemically modified carbon. Different forms of carbonaceous material which are at least partially crystalline can be characterized by their respective degrees of graphitization. The term "degree of graphitization" refers to the value g according to the formula: ##EQU1## wherein d.sub.002 represents the spacing (.ANG.) between the graphitic layers of the carbonaceous material in the crystal structure, determined by standard X-ray diffraction techniques, and g represents a dimensionless number with a value between 0 and 1.0. In general, carbonaceous material having a high degree of graphitization, for example graphite, has a more ordered microstructure, whereas carbonaceous material having a low degree of graphitization, for example coke, has a less ordered microstructure. A high degree of graphitization in the carbonaceous material of the anode provides a higher cell capacity in conjunction with less variation of cell voltage.
The voltage profile, reversibility and final stoichiometry of the alkali metal-intercalated carbonaceous material is dependent on the structure of the carbonaceous material. For example, petroleum coke has a turbostratic structure, shows a steep voltage profile, and intercalates up to a stoichiometry of Li.sub.0.5 C.sub.6. On the other hand, graphite has a nearly perfect layered structure and is able to intercalate up to a stoichiometry of LiC.sub.6, with a flat voltage curve near zero volts relative to lithium. The theoretical capacity of a graphite anode is 372 mAh/g based on the stoichiometry of LiC.sub.6, thereby making graphite one of the most desirable candidates for a carbonaceous anode material (Shu, Z. X. et al "Electrochemical Intercalation of Lithium into Graphite" J Electrochem Soc 140: 4: 922-927; 1993).
Canadian Patent Number 1,265,580 (Yoshino, A. et al, Feb. 6, 1990) discloses a secondary cell wherein the anode is made of carbonaceous material into which alkali metal ions may be intercalated reversibly and the cathode is made of an active material consisting of a sulfide or an oxide of a transition metal. Highly graphitic carbonaceous materials such as graphite are inexpensive, non-toxic and are capable of incorporation into secondary cells having relatively high specific capacities.
However, there are numerous problems associated with the use of a carbonaceous anode. In particular, compounds of the formula M.sub.x C are reactive materials which are difficult to handle in air. Accordingly, the anode is preferably produced in situ in a cell by an initial intercalation step. However, some of the alkali metal ions and the electrolyte are consumed in an irreversible reaction in the initial intercalation step. This irreversible reaction results in an initial capacity loss for the cell which reduces the overall performance thereof.
Without being bound by theory, it is believed by those skilled in the art that the reaction which occurs during the initial intercalation step involves the formation of a passivation film on the bare surfaces of the carbonaceous material by decomposition of electrolyte salt and/or solvent. The ideal passivation film is insoluble in the electrolyte and is an electronic insulator and an ionic conductor for alkali metal ions and thus protects the electrolyte from decomposition on bare carbonaceous material surfaces. However, while the passivation film is being formed, additional bare surfaces of the carbonaceous material can be exposed to electrolyte. This is believed by some (for example, Wilkinson, D. P. et al, U.S. Pat. No. 5,130,211 issued Jul. 14, 1992) to occur by co-intercalation of electrolyte solvent leading to exfoliation and thus an increase in the surface area of the carbonaceous material.
It is believed that bare surfaces can also be exposed by at least two other methods. Firstly, the passivation film may be ruptured by the formation of gaseous products arising from the reduction of electrolyte solvent by intercalated carbonaceous material (Shu, Z. X. et al "Effect of 12 Crown 4 on the Electrochemical Intercalation of Lithium into Graphite" J Electro Soc 140: 6: L101-L103; 1993) and secondly, by changing the layer spacing of the carbonaceous material, in particular graphite, as alkali metal ions intercalate therein. This change in the layer spacing can rupture the passivation film exposing bare surfaces on the carbonaceous material. Layer spacing changes can also occur during alkali metal ion de-intercalation from carbonaceous material resulting in further passivation film rupture, bare surface exposure and the formation of a new passivation film on the resultant bared or partially covered surfaces.
Regardless of whether electrolyte solvent is co-intercalated, gaseous products are formed or the layer spacing is changed, intercalation and de-intercalation of alkali metal ions cause continual exposure of bare surfaces to electrolyte, thereby resulting in continual consumption of electrolyte in the formation of new passivation films. Thus, the passivation film is not stable to repeated intercalation and de-intercalation of alkali metal ions into the carbonaceous material during discharge/charge cycles so that concomitant electrolyte consumption and passivation film growth occurs. If this process continues unabated, then, with repeated cell discharge/charge cycles, the internal impedance of the cell will increase and, between fixed cell voltage limits, will result in progressive loss of capacity. Accordingly, it becomes difficult to attain the theoretical capacity of the carbonaceous material, which for example is thought to be 372 mAh/g for graphite. This progressive loss is commonly referred to by those skilled in the art as "capacity fade".
Accordingly, there is a requirement for an electrolyte which is capable of forming a stable passivation film using as little electrolyte as possible.
U.S. Pat. No. 5,028,500 (Fong, R. et al, Jul. 2, 1991) describes two solutions to prevent excessive electrolyte decomposition at highly graphitic carbonaceous intercalation hosts and the consequent loss of cell capacity and performance properties. The first is to form a dual phase carbonaceous intercalation host having a mean degree of graphitization of at least 0.40, the first phase having a degree of graphitization greater than 0.40 and the second phase having a degree of graphitization less than 0.40. The other approach maintains the carbonaceous intercalation host at a temperature greater than about 50.degree. C. during the initial intercalation of the host with lithium. Both approaches reduced electrolyte decomposition during the initial intercalation step, thereby improving the cell performance and capacity. However, a solution to electrolyte decomposition that does not require the replacement of single-phase highly graphitic intercalation hosts or heat treatment would be highly desirable.
Excessive electrolyte decomposition during the initial intercalation step of a highly graphitic carbonaceous intercalation host has also been suppressed by addition of a sequestering agent, such as a crown ether, as disclosed in U.S. Pat. No. 5,130,211 (Wilkinson, D. P. et al, Jul. 14, 1992). The crown ether chelates with alkali metal and prevents both electrolyte solvent co-intercalation into the carbonaceous host and exfoliation of the carbonaceous host. Hence, the increase in surface area of carbonaceous materials is prevented and excessive electrolyte decomposition is reduced.
The use of crown ether as a sequestering agent was further explored by Shu, Z. X. et al ("Effect of 12 Crown 4 on the Electrochemical Intercalation of Lithium into Graphite" J Electro Soc 140: 6: L101-L103; 1993). The crown ether was found to suppress electrolyte solvent decomposition by minimizing formation of gaseous products which, as discussed hereinabove, is believed to result in exposure of bare surfaces to the electrolyte, consumption of electrolyte and formation of a new passivation film on the bared or partially covered surfaces. However, crown ethers are expensive and generally highly toxic. Accordingly, there is a requirement for a viable alternative to the use of crown ethers.
It is an object of the present invention to improve the performance and the overall capacity of secondary cells by minimizing electrolyte decomposition. It is another object of the present invention to reduce the capacity loss during the initial intercalation step.